1. Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China;
2. School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907
3. Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China
4. School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907 e-mail: